1. Field of the Invention
This invention relates to the analysis of fluid flows inside hollow bodies such as, for example, engines, test rigs and pipes, and other bodies in which fluids may flow such as river estuary models. The invention may be used to investigate liquid, gaseous or two phase fluid flows.
2. Description of the Related Art
The use of X-Y position sensitive, or two-dimensional (2D) radiation "cameras" to trace the distribution of radioisotope-labelled liquids introduced into the human body is now a well-established aid to diagnosis of certain medical conditions. One such technique involves the use of a positron-emitting isotope. Each emitted positron travels a certain distance before coming to rest (i.e. to thermal equilibrium with the surroundings). It then annihilates with an electron and produces two 511 keV photons which are emitted in almost exactly opposite directions. Tomography is possible, using a 2D detector on each side of the subject and a small computer to receive, construct, and display the image. The desired radiation is discriminated from background by the acceptance of only the coincident events in the detectors produced by the two photons emitted in almost exactly opposite directions.
With the positron emission tomography (PET) technique the displayed image is computed from the recorded information for a selected plane in the subject. Only events due to decays in this plane appear in focus and, such as is the power of the technique, decays in other planes appear as a more or less uniform background. Moreover, as any of the planes between the detectors may be selected, this technique inherently yields three-dimensional information about its subject.
Our patent specification No. GB 2087685 (corresponding to U.S. application Ser. No. 318,939 filed Nov. 6, 1981) describes a method and apparatus enabling similar studies in engineering applications, such as studying fluid flow in gas turbine engines, where the requirements are much more rigorous. According to that specification, a method of producing a representation of a fluid flow inside a hollow body comprises the following steps:
injecting into the fluid a quantity of a radioactive isotope label which is compatible with the fluid,
detecting radiation emitted due to the radioactive decay of the isotope by detection apparatus located outside the body and which produces output signals responsive to the detected radiation,
generating a representation of the spatial distribution of the structure of the body which surrounds the fluid flow and of the material from which it is made,
producing from said representation attenuation signals representative of the attenuation of the radiation passing through the body,
adjusting the output signals from the detection apparatus in dependence upon the attenuation signals to compensate for the attenuation of the radiation and providing adjusted output signals, and,
producing from said adjusted output signals a representation of the fluid flow inside the body.
The radioactive isotope label is injected into the fluid in the system under study in a compatible carrier fluid, generally a liquid. In the preferred embodiment, decay of the isotope produces positrons, which when near rest combine with electrons in the fluid. Each positron thus annihilated, produces gamma rays (photons) which can be detected.
Patent Specification No. GB 2031142 A describes a medical use of positron emission tomography, in which very small (6.mu.-8.mu.) solid carrier particles are labelled with a positron-emitting isotope and injected into a patient's bloodstream. The particles are tracked in three dimensions. As with all the above noted prior work, however, the positrons travel some distance in the surrounding tissue before coming to rest and emitting two gamma rays. Particularly if the fluid under study is gaseous, the mean free path of a positron before it nears thermal equilibrium with the gas, combines with an electron and is annihilated may be relatively long. Since the gamma rays which are detected indicate the position at which annihilation takes place, rather than the position at which the corresponding isotope atom decayed, this results in a reduction in clarity. The tomograph produced is based not on the true position at which the positron was produced, but on the position at which the positron annihilated. The resulting image has a degree of "fuzziness" corresponding to the distances and random directions travelled by the various positrons whose annihilation is detected. In some cases, especially if the fluid is gaseous, the positrons may tend to reach the walls of the hollow body containing the fluid before annihilation, so that the resulting representation merely shows up the walls. This can still be useful in very narrow passages, to indicate if gas is flowing, but means that the technique cannot show gas flows in larger hollow bodies. Such problems (of a relatively long positron path before annihilation) can also be particularly acute if one wishes to study the flow of a fine liquid jet into a gaseous atmosphere.